Structural color originates from the selective reflection of particular light wavelengths through constructive and destructive interference rather than absorption. The magnificent and brilliant colors found in nature originate from the interference of light within nanoscale periodic structures.1-3 Most of the reflectors found in nature arise from the interference of light on stacks of thin films comprised of alternating low and high refractive index materials.1,2 By appropriately selecting materials with refractive index contrast, one can build multiple stack films in order to mimic the brilliant colors found in nature. These films may be reduced to flakes and incorporated into a paint or coating system,4,5 and they can also be used as a coating on glass windows to prevent bird collision6-8.
The coloring mechanism based on light absorption, common in pigments, usually leads to a broad wavelength reflection with reflectivity around 50-60%. In contrast, light reflected from periodic nanostructures may achieve up to 100% reflectivity of a pure color.8 The common coating systems, based on light absorption, are prone to photo-degradation and thermal oxidation when exposed to sun light; therefore, UV absorbers are usually added to the coating in order to increase its durability.9 In principal, color based on a nanostructure material will not deteriorate under sun light exposure, since the coloring mechanism does not involve absorption and there are no polymers in the coating composition.
Bragg stacks (1D photonic crystals) with structural color and superhydrophilic and self-cleaning properties were reported by Wu et al.10 with narrow range reflectance in the visible and near UV region obtained using a non-quarter wave design to build the multiple stack arrays. Kurt et al.8 also reported that structural colors can be obtained via LbL assembly by alternating layers of high and low refractive index materials. Stacks with low refractive index were assembled with SiO2 nanoparticles and Poly(allylamine hydrochloride) (PAH), and high refractive index stacks were assembled using TiO2 nanoparticles and Poly(vinyl sulfonic acid) (PVS). Calcination was applied to remove the polymers after the deposition of each stack. Nanoporous stacks of TiO2 and SiO2 nanoparticles were successively deposited on top of each other until the optimum reflectance at a selected wavelength was achieved. By properly selecting nanoparticles size and experimental parameters, Kurt and co-workers were able to create coatings with tunable reflectance in the visible (structural colors) and near-UV light wavelengths.
The LbL technique, used by Wu and Kurt to create Bragg reflectors, has several advantages for thin films processing, such as low cost and conformal coating on several types of substrates.11 However, this technique can be limited by the time required for polyelectrolyte assembly and, depending on the application, by the size or type of the substrate.
Krogman et al.12-14 have disclosed an automated system for depositing thin polymer films from atomized mists of solutions containing species of complementary functionality. However, such a spray technique has not heretofor been used for making a multilayer structure having alternating and predetermined indices of refraction such that a structural color with ultraviolet reflectance is provided. As such, a spray LbL process that provides time efficient assembly of structural colors with less constraint on size and type of substrate would be desirable.